Class and Feature names

The first thing to do is to declare the name of the built-in feature and the name of its class. Let's assume that the built-in function foo and built-in procedure poo have been declared in class BAR. These feature and class names will have to be declared as follows in class ET_TOKEN_CONSTANTS if not already done:

feature -- Class names

	bar_class_name: ET_CLASS_NAME
			-- "BAR" class name
		once
			create {ET_IDENTIFIER} Result.make (capitalized_bar_name)
		ensure
			bar_class_name_not_void: Result /= Void
		end

feature -- Feature names

	foo_feature_name: ET_FEATURE_NAME
			-- 'foo' feature name
		once
			create {ET_IDENTIFIER} Result.make (foo_name)
		ensure
			foo_feature_name_not_void: Result /= Void
		end

	poo_feature_name: ET_FEATURE_NAME
			-- 'poo' feature name
		once
			create {ET_IDENTIFIER} Result.make (poo_name)
		ensure
			poo_feature_name_not_void: Result /= Void
		end
		
feature -- Keyword and symbol names

	capitalized_bar_name: STRING = "BAR"
			-- Name of Eiffel class "BAR"

	foo_name: STRING = "foo"
			-- Name of Eiffel feature 'foo'

	poo_name: STRING = "poo"
			-- Name of Eiffel feature 'poo'

It is a good practice to keep these features in alphabetic order in their respective feature clauses. Note that capitalized_bar_name, foo_name and poo_name also need to be listed in feature strings from class ET_EIFFEL_SCANNER_SKELETON:

	strings: DS_HASH_TABLE [INTEGER, STRING]
			-- Strings known by the current scanner, and the associated
			-- hash codes when they are used as identifier
		once
			...
				-- Class names.
			Result.force_new (-1, tokens.capitalized_bar_name)
			...
				-- Feature names.
			Result.force_new (-1, tokens.foo_name)
			...
			Result.force_new (-1, tokens.poo_name)
			...
		end

Again, try to keep them in alphabetic order in their respective sections.

Built-in feature codes

Each built-in feature is associated with a code which is made up of a class identifier and a feature identifier within this class. Still assuming that the built-in function foo and the built-in procedure poo have been declared in class BAR, the following features need to be declared in class ET_TOKEN_CODES if not already done:

feature -- Built-in codes

	...
	builtin_bar_class: NATURAL_8 = XX
	...
			-- Code for classes of built-in features
	
	builtin_bar_foo: NATURAL_8 = YY
			-- Codes for built-in functions from class "BAR"

	builtin_bar_poo: NATURAL_8 = ZZ
			-- Codes for built-in procedures from class "BAR"

where XX is the class identifier whose value should be different from all other builtin_*_class declared in this feature clause, and YY and ZZ are unique identifiers within the built-in features of this class. Note that the identifiers for the built-in functions should be listed first, followed by the identifiers for the built-in procedures.

This convenience routine should also be added in class ET_DYNAMIC_FEATURE:

	is_builtin_bar_class: BOOLEAN
			-- Is current feature the built-in feature of class "BAR"?
		do
			Result := builtin_class_code = {ET_TOKEN_CODES}.builtin_bar_class
		ensure
			builtin: Result implies is_builtin
			definition: Result = (builtin_class_code = {ET_TOKEN_CODES}.builtin_bar_class)
		end

Validity checker

When features are flattened, the Eiffel compiler checks whether features declared as built-in are known and if they have the expected signature. This is done in class ET_BUILTIN_FEATURE_CHECKER. Let's assume that the feature foo is a built-in function with no argument and a result of type 'like Current', and the feature poo is a built-in procedure with one argument of type INTEGER. The following features need to be modified or added in class ET_BUILTIN_FEATURE_CHECKER:

	check_builtin_feature_validity (a_feature: ET_EXTERNAL_ROUTINE)
			-- Check validity of built-in `a_feature', written in `current_class'.
			-- Set `has_fatal_error' if a fatal error occurred.
		require
			a_feature_not_void: a_feature /= Void
		local
			l_name: ET_CLASS_NAME
		do
			l_name := current_class.name
			if ... then
			...
			elseif l_name.same_class_name (tokens.bar_class_name) then
				check_builtin_bar_feature_validity (a_feature)
			...
			else
					-- Unknown built-in routine.
				a_feature.set_builtin_code (tokens.builtin_unknown)
				if unknown_builtin_reported then
					set_fatal_error
					error_handler.report_gvkbu1a_error (current_class, a_feature)
				end
			end
		end

	check_builtin_bar_feature_validity (a_feature: ET_EXTERNAL_ROUTINE)
			-- Check validity of built-in `a_feature' from class "BAR".
			-- Set `has_fatal_error' if a fatal error occurred.
		require
			a_feature_not_void: a_feature /= Void
		local
			l_builtin_features: DS_HASH_TABLE [TUPLE [arguments: detachable ARRAY [ET_TYPE]; type: detachable ET_TYPE; builtin_feature_code: NATURAL_8], ET_FEATURE_NAME]
			l_builtin_class_code: NATURAL_8
		do
			l_builtin_class_code := tokens.builtin_bar_class
			builtin_features.search (l_builtin_class_code)
			if builtin_features.found then
				l_builtin_features := builtin_features.found_item
			else
				create l_builtin_features.make_map (...)
				l_builtin_features.set_key_equality_tester (feature_name_tester)
				builtin_features.force_last (l_builtin_features, l_builtin_class_code)
					-- Functions.
				...
				register_builtin_feature (tokens.foo_feature_name, Void, tokens.like_current, tokens.builtin_bar_foo, l_builtin_features)
				...
					-- Procedures.
				...
				register_builtin_feature (tokens.poo_feature_name, <<current_universe.integer_type.type>>, Void, tokens.builtin_bar_poo, l_builtin_features)
				...
			end
			check_expected_builtin_feature_validity (a_feature, l_builtin_class_code, l_builtin_features)
		end

Various examples with different combinations of argument and result types can be found in class ET_BUILTIN_FEATURE_CHECKER.

Dynamic type sets

The Gobo Eiffel compiler uses the dynamic type sets of the target of the feature calls to implement dynamic binding and CAT-call checking. Each expression is associated with a dynamic type set which is then propagated when there is an assignment or argument passing. The dynamic type set of the 'Result' entity of built-in functions needs to be built. If it is only made up of exactly one type which is the declared type of the 'Result' entity of the built-in function, then nothing special needs to be done. The default implementation will take care of this case. On the other hand, if the dynamic type set is more complicated or if the built-in feature calls internally another feature whose dynamic type sets need to be updated as well (for example feature twin calls feature copy), then the following code needs to appear in class ET_DYNAMIC_TYPE_BUILDER:

feature {NONE} -- Feature validity

	check_external_builtin_function_validity (a_feature: ET_EXTERNAL_FUNCTION)
			-- Check validity of `a_feature'.
			-- `a_feature' is a built-in function.
			-- Set `has_fatal_error' if a fatal error occurred.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_builtin: a_feature.is_builtin
			builtin_feature_known: not a_feature.is_builtin_unknown
		do
			inspect a_feature.builtin_class_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_class then
				inspect a_feature.builtin_feature_code
				when {ET_TOKEN_CODES}.builtin_bar_foo then
					report_builtin_bar_foo (a_feature)
				else
					report_builtin_function (a_feature)
				end
			...
			else
				report_builtin_function (a_feature)
			end
		end

feature {NONE} -- Built-in features

	report_builtin_bar_foo (a_feature: ET_EXTERNAL_FUNCTION)
			-- Report that built-in feature 'BAR.foo' is being analyzed.
		require
			no_error: not has_fatal_error
			a_feature_not_void: a_feature /= Void
		local
			l_result_type: ET_DYNAMIC_TYPE
		do
			if current_type = current_dynamic_type.base_type then
				...
				l_result_type := result_type_set.static_type
				propagate_builtin_result_dynamic_types (l_result_type, current_dynamic_feature)
				...
			end
		end

By default, nothing needs to be done for built-in procedures. But there are some special cases when the built-in procedure will call internally another feature or will have an effect on the dynamic type set of some entities (for example in class SPECIAL, the procedure put will have an effect on the dynamic type set of item). In this case, code similar to the one above will be needed for these built-in procedures. Examples can be found in class ET_DYNAMIC_TYPE_BUILDER.

Generation of C code

The last thing to do is to make sure that the C code of the built-in feature is properly generated. Still assuming that foo is a built-in function and poo is a built-in procedure declared in class BAR. Then the following code needs to appear in class ET_C_GENERATOR:

feature {NONE} -- Feature generation

	print_external_builtin_function_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in function.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_function: a_feature.is_function
			a_feature_is_builtin: a_feature.is_builtin
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_class_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_class then
				print_external_builtin_bar_function_body (a_feature)
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

	print_external_builtin_bar_function_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in function introduced in class "BAR".
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_function: a_feature.is_function
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: current_feature.is_builtin_bar_class
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_feature_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_foo then
				print_builtin_bar_foo_body (a_feature)
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

	print_external_builtin_procedure_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in procedure.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_procedure: a_feature.is_procedure
			a_feature_is_builtin: a_feature.is_builtin
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_class_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_class then
				print_external_builtin_bar_procedure_body (a_feature)
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

	print_external_builtin_bar_procedure_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in procedure introduced in class "BAR".
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_procedure: a_feature.is_procedure
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: current_feature.is_builtin_bar_class
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_feature_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_poo then
				print_builtin_bar_poo_body (a_feature)
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

feature {NONE} -- Built-in feature generation

	print_builtin_bar_foo_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of `a_feature' corresponding
			-- to built-in feature 'BAR.foo'.
		require
			a_feature_not_void: a_feature /= Void
			valid_feature: current_feature.static_feature = a_feature
		do
			...
		end

	print_builtin_bar_poo_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of `a_feature' corresponding
			-- to built-in feature 'BAR.poo'.
		require
			a_feature_not_void: a_feature /= Void
			valid_feature: current_feature.static_feature = a_feature
		do
			...
		end

In some cases, the C code can be inlined. This is when the call to the C function can be avoided by calling the C code directly. In that case the code above in class ET_C_GENERATOR can be changed by replacing print_builtin_bar_foo_body by print_builtin_bar_foo_call and print_builtin_bar_poo_body by print_builtin_bar_poo_call, and by modifying features print_external_builtin_bar_function_body and print_external_builtin_bar_procedure_body as follows:

feature {NONE} -- Feature generation

	print_external_builtin_bar_function_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in function introduced in class "BAR".
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_function: a_feature.is_function
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: current_feature.is_builtin_bar_class
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_feature_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_foo then
				fill_call_formal_arguments (a_feature)
				print_indentation_assign_to_result
				print_builtin_bar_foo_call (current_feature, current_type, False)
				print_semicolon_newline
				call_operands.wipe_out
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

	print_external_builtin_bar_procedure_body (a_feature: ET_EXTERNAL_ROUTINE)
			-- Print to `current_file' the body of built-in feature `a_feature'.
			-- `a_feature' is a built-in procedure introduced in class "BAR".
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_procedure: a_feature.is_procedure
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: current_feature.is_builtin_bar_class
			valid_feature: current_feature.static_feature = a_feature
		do
			inspect a_feature.builtin_feature_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_poo then
				fill_call_formal_arguments (a_feature)
				print_builtin_bar_poo_call (current_feature, current_type, False)
				call_operands.wipe_out
			...
			else
					-- Internal error: unknown built-in feature.
					-- This error should already have been reported in ET_FEATURE_FLATTENER.
				set_fatal_error
				error_handler.report_giaaa_error
			end
		end

feature {NONE} -- Query call generation

	print_builtin_query_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call to query `a_feature' (static binding).
			-- `a_feature' is a built-in feature.
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
			-- Note that the result of the query is not adapted to match the kind
			-- of result type expected by the caller. It is recommended to use
			-- `print_adapted_query_call' whenever possible.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_query: a_feature.result_type_set /= Void
			a_feature_is_builtin: a_feature.is_builtin
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			inspect a_feature.builtin_class_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_class then
				print_builtin_bar_query_call (a_feature, a_target_type, a_check_void_target)
			...
			else
				print_non_inlined_query_call (a_feature, a_target_type, a_check_void_target)
			end
		end

	print_builtin_bar_query_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call to query `a_feature' (static binding).
			-- `a_feature' is a built-in feature introduced in class "BAR".
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
			-- Note that the result of the query is not adapted to match the kind
			-- of result type expected by the caller. It is recommended to use
			-- `print_adapted_query_call' whenever possible.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_query: a_feature.result_type_set /= Void
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: a_feature.is_builtin_bar_class
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			inspect a_feature.builtin_feature_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_foo then
				print_builtin_bar_foo_call (a_feature, a_target_type, a_check_void_target)
			...
			else
				print_non_inlined_query_call (a_feature, a_target_type, a_check_void_target)
			end
		end

feature {NONE} -- Procedure call generation

	print_builtin_procedure_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call to procedure `a_feature' (static binding).
			-- `a_feature' is a built-in feature.
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_builtin: a_feature.is_builtin
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			inspect a_feature.builtin_class_code
			...
			when {ET_TOKEN_CODES}.builtin_bar_class then
				print_builtin_bar_procedure_call (a_feature, a_target_type, a_check_void_target)
			...
			else
				print_non_inlined_procedure_call (a_feature, a_target_type, a_check_void_target)
			end
		end

	print_builtin_bar_procedure_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call to procedure `a_feature' (static binding).
			-- `a_feature' is a built-in feature introduced in class "BAR".
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
		require
			a_feature_not_void: a_feature /= Void
			a_feature_is_builtin: a_feature.is_builtin
			a_feature_is_builtin_bar: a_feature.is_builtin_bar_class
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			inspect a_feature.builtin_feature_code
			when builtin_bar_poo then
				print_builtin_bar_poo_call (a_feature, a_target_type, a_check_void_target)
			...
			else
				print_non_inlined_procedure_call (a_feature, a_target_type, a_check_void_target)
			end
		end

feature {NONE} -- Built-in feature generation

	print_builtin_bar_foo_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call (static binding) to `a_feature'
			-- corresponding to built-in feature 'BAR.foo'.
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
		require
			a_feature_not_void: a_feature /= Void
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			...
		end

	print_builtin_bar_poo_call (a_feature: ET_DYNAMIC_FEATURE; a_target_type: ET_DYNAMIC_TYPE; a_check_void_target: BOOLEAN)
			-- Print to `current_file' a call (static binding) to `a_feature'
			-- corresponding to built-in feature 'BAR.poo'.
			-- `a_target_type' is the dynamic type of the target.
			-- `a_check_void_target' means that we need to check whether the target is Void or not.
			-- Operands can be found in `call_operands'.
		require
			a_feature_not_void: a_feature /= Void
			a_target_type_not_void: a_target_type /= Void
			call_operands_not_empty: not call_operands.is_empty
		do
			...
		end